1. Field of the Invention
The present invention pertains to a method and apparatus for monitoring the integrity of wires or electrical cables, or the like, including but not limited to detecting faults or defects in the same.
2. Related Background Art
Various methods for monitoring the integrity of wires, such as detecting faults in a wire, are known. One such method is time domain reflectometry (TDR). TDR operates as follows. First, a fast rise time pulse is injected into a wire. Then, if the pulse encounters an impedance which is not equal to the source impedance of the TDR pulse generator, such as may be caused by a fault in the wire, the pulse is reflected back toward the injection point. Finally, the reflection of the pulse is captured by the TDR instrument and is measured and analyzed. The reflected voltage waveform represents the distributed impedance of the wire under test as a function of time. Where there is a fault in the wire under test, the normal impedance of the wire is changed, resulting in a mismatch between the impedance at the fault location and the impedance at a non-fault location. This mismatch will appear as a noticeable change in the reflected voltage waveform. Thus the voltage waveform (the TDR “signature”) serves to detect faults.
More specifically, TDR signatures can be used to determine the nature, magnitude and location of a fault. The nature of the change in the TDR signature, for example, whether the change is an increase or a decrease in voltage, may indicate the nature of the fault, for example, whether the fault is an open circuit or a short circuit. The magnitude of the change in the voltage waveform is proportional to the magnitude of the fault. The location of the fault may be determined as follows. When the pulse output by the TDR reaches the fault, a portion of the TDR pulse is reflected back to the TDR. The speed at which the TDR pulse travels along the wire can be determined either by empirically measuring it by using TDR on a wire of known length, or by referring to standard tables available from wire manufacturers. The time at which the TDR pulse is sent down the wire and the time at which the reflected portion of the pulse is received back at the injection point can be monitored. The location of the fault (the distance along the wire from the injection point to the fault) can then be calculated from the speed and round-trip travel time of the pulse.
Another known method for monitoring wire integrity involves the measurement of the dissipation factor (DF) of the wire. DF is a function of the wire's impedance, Z. Specifically,Z=R±j*X, whereR=the real part of the impedance=AC resistance;X=the imaginary part of the impedance=reactance; andDF=R/X.
For example, the DF of an insulated wire may be measured upon the application of an AC voltage thereto. The AC voltage impresses an alternating electric field on the wire under test, exciting the wire under test. This alternating electric field causes any polar dielectric molecules in the dielectric insulation material, such as undesirable water molecules, to also alternate (to align with the field), resulting in the dissipation of energy of the applied field. In the ideal case, no energy of the applied electric field would be dissipated, the AC resistance would be zero, and the DF would also be zero. But in wires containing water or some other polar molecules in the dielectric insulation material, some energy is dissipated, resulting in a non-zero AC resistance and a measurable DF. Further, as insulation materials age, they become less supple and pliable due to a variety of environmental effects. Increased brittleness, in turn, results in greater AC resistance of the molecular motion described above, and hence greater dissipation and higher DF measurements. Thus, measuring the DF value, or the trend of DF values over time, of a wire under test, and comparing these with the DF value for a reference wire of the same type in good condition, provides a useful means for monitoring the physical integrity of wiring and detecting losses in this integrity, such as faults.
The value of DF depends principally on the properties of the dielectric material, the condition of the dielectric material, and the frequency of the applied voltage. Generally, for wire insulation, the smaller the value of DF (i.e. the less energy dissipated), the more effective the insulation material will be. Note that air can also serve as a dielectric (i.e., an uninsulated wire)
Existing fault-detection technology suffers from a number of problems. For example, currently known methods are often inadequate to detect small defects in a wire, such as nicks, chafing, and poor connector contacts, because existing technology is not sensitive enough and yields results (e.g. TDR signatures) that are insufficiently precise. But these types of small defects are of particular importance because, while they do not impair operation of the wiring, they have the potential to become full-fledged faults in the wiring that could cause a severe malfunction while the device connected to the wiring is in operation. Indeed, the very fact that these small defects do not yet impair operation may hinder their detection and thereby exacerbate the problem.
Many of the current problems in fault-detection technology are of particular interest for those in the aircraft industry, an important customer of this technology. Defects such as nicks, chafing, and poor connector contacts are common in aircraft wiring, due, for example, to mechanical abrasion, fluid contamination (by water, hydraulic fluid or other fluids used in aircraft), or heat stress. Also, the particular types of wire insulation commonly used on aircraft are known to acquire similar kinds of small defects as they age. For example, polyimide (Kapton®) insulated wire is known to develop radial cracks over time. Initially, the crack may have no effect on the wire's operation, but it could gradually grow to encircle the entire wire, resulting eventually in a bare, uninsulated wire. It is crucial for aircraft maintenance and repair personnel to detect these defects before they become full-fledged faults, so as to prevent malfunctions of aircraft components that could occur while the aircraft is in operation.
In addition, aircraft generally employ wires which are short and unshielded. Both of these characteristics of aircraft wiring pose difficulties for TDR. TDR is known to have problems in accurately locating a fault a short distance from the injection point, because errors in measurement of the travel time of the TDR pulse have a more significant effect. Again, the non-uniform geometry of unshielded wires yields a less smooth waveform (TDR signature), which is more difficult to interpret.
Another limitation of conventional fault-detection technology is that it requires the use of a baseline or reference value obtained from a faultless wire. For example, conventional TDR testing requires a baseline TDR signature as a reference against which the TDR signatures acquired in actual testing may be compared. Thus wires which are expected to be put into use and hence to require future testing while in use should be tested prior to use, when they are in a virgin state (i.e., pristine condition), in order to obtain the baseline signatures. However, the acquisition of this data is not always possible. There may be wires already in use which need to be tested and for which no baseline data was obtained. Even if baseline data is subsequently obtainable, it may be time-consuming to obtain it.